1. Field of the Invention
Embodiments of the present invention relate to a flat panel display, and more particularly, to a thin film transistor substrate for a flat panel display and a method of manufacturing the same.
2. Description of the Related Art
Flat panel displays such as liquid crystal displays (LCDs) and organic light emitting displays (OLEDs) include a thin film transistor substrate, on which a thin film transistor functioning as a switching device is formed, for active matrix driving. The liquid crystal display generally displays an image using electrooptic characteristics of liquid crystal molecules in a liquid crystal layer.
The liquid crystal display includes a color filter substrate and a thin film transistor substrate which face each other with the liquid crystal layer sandwiched therebetween. The color filter substrate allows an image displayed on a liquid crystal panel to have color. The thin film transistor substrate includes a thin film transistor functioning as a switching device, thereby applying a data voltage provided by a driving circuit to the liquid crystal layer.
The thin film transistor includes an ohmic contact layer, a gate electrode, a source electrode, a drain electrode, and an active layer, and the active layer forms a channel of the thin film transistor. The thin film transistor is typically manufactured using a 5-photomask process. However, the manufacturing cost of the thin film transistor is high due to the use of a 5-photomask process. Accordingly, a 4-photomask process is used to reduce the manufacturing cost.
The 4-photomask process includes a first photomask process for forming the gate electrode and a gate line, a second photomask process for forming a gate insulating layer, an ohmic contact pattern, the active layer, the source electrode, the drain electrode, and a data line, a third photomask process for forming a contact hole exposing portions of a passivation layer and the drain electrode, and a fourth photomask process for forming a pixel electrode. However, in a case of using the 4-photomask process, since the active layer, the source electrode, the drain electrode, and the data line are simultaneously formed using one partial exposure mask, the active layer underlying the data line protrudes from the data line. This results in the occurrence of a wavy noise. The wavy noise is caused by interference generated between the active layer and the pixel electrode when leakage current in the active layer occurs due to light form a backlight unit. The wavy noise generates moiré fringe on an image displayed on the liquid crystal panel, thereby reducing the display quality of the liquid crystal display.
When the active layer, the source electrode, the drain electrode, and the data line are formed using one partial exposure mask in the four photomask process, the active layer can be over-etched such that the active layer may be formed in a back channel structure. Accordingly, the active layer needs to be thick to secure a margin for an etching process of the active layer such that the characteristics (for example, mobility and sub-threshold) of the thin film transistor are not negatively affected. A method for forming an etch stopper on the active layer can be used to prevent such problems. However, a separate photomask process for forming the etch stopper is added such that there is again the problem of a complicated five photomask process, which increases manufacturing cost of the thin film transistor substrate.